1. Field of the Invention
The present invention relates to an apparatus for producing a carbon nanotube fiber and a method for producing a carbon nanotube fiber using the apparatus.
2. Description of the Related Art
Carbon nanotubes (CNTs) are allotropes of carbon that have a diameter of several to tens of nanometers and a length of hundreds of micrometers to several millimeters. Since the synthesis of carbon nanotubes was first reported in Nature by Dr. Iijima in 1991, carbon nanotubes have been investigated in various fields due to their excellent thermal, electrical and physical properties and high length-to-diameter ratio. Such inherent characteristics of carbon nanotubes are attributed to the sp2 bonding of carbon. Carbon nanotubes are stronger than steel and lighter than aluminum, and exhibit high electrical conductivity comparable to metals. Carbon nanotubes can be broadly classified into single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTs), and multi-wall carbon nanotubes (MWNTs) depending on how many they have walls. Carbon nanotubes are divided into zigzag, armchair, and chiral conformations depending on their asymmetry/chirality.
Most of the studies to date have been directed towards the use of dispersions of carbon nanotubes in the form of powders as reinforcing agents of composite materials and the production of transparent conductive films using dispersions of carbon nanotubes. Carbon nanotubes have already reached commercialization in some applications. Dispersion of carbon nanotubes is important for the use of the carbon nanotubes in composite materials and transparent conductive films. However, high cohesive strength of carbon nanotubes due to their strong van der Waals force makes it not easy to disperse the carbon nanotubes at a high concentration while maintaining their dispersibility. Further, in composite materials using carbon nanotubes as reinforcing agents, the excellent characteristics of the carbon nanotubes are not sufficiently exhibited.
Under these circumstances, a great deal of research on the processing of carbon nanotubes into fibers has recently been conducted to produce carbon nanotube structures that sufficiently exhibit the characteristics of carbon nanotubes. Methods for processing carbon nanotubes into fibers can be classified into two methods: wet and dry methods.
An example of the wet methods is coagulation spinning. According to the coagulation spinning, when a dispersion containing carbon nanotubes and a dispersant is introduced into a polymer solution, the dispersant moves from the dispersion to the polymer solution and the polymer enters the dispersion to replace the dispersant, acting as a binder. The carbon nanotube fiber thus produced includes about 60% by weight of the carbon nanotubes. However, the physical properties of the carbon nanotube fiber are not satisfactory despite the significantly higher content of the carbon nanotubes than the content of carbon nanotubes in existing composite materials.
Another wet method for processing carbon nanotubes into fibers is liquid-crystalline spinning taking advantage of the ability of solutions of carbon nanotubes to form liquid crystals under specified conditions. This method is advantageous in that fibers of well-aligned carbon nanotubes can be produced, but has the disadvantages of very low spinning speed and strict conditions for the formation of liquid crystals of carbon nanotubes (S. Zhang, K. K. Koziol, I. A. Kinloch, A. H. Windle, “Macroscopic Fibers of Well-Aligned Carbon Nanotubes by Wet Spinning”, Small 4, 1217 (2008)).
One of the dry methods for producing carbon nanotube fibers is brush spinning in which carbon nanotubes grown vertically on a silicon wafer are twisted and fiberized. However, the limited size of the silicon wafer causes many difficulties in large-scale production (K. Jiang, Q. Li, and S. Fan, “Spinning continuous carbon nanotube yarns”, Nature 419, 801 (2002)).
Another dry method is direct spinning proposed by Professor Windle. As illustrated in FIG. 1, a carbon source and a catalyst, together with a carrier gas, are introduced into a vertically standing furnace through an inlet formed at the top of the furnace, carbon nanotubes are synthesized in the furnace, aggregates of the carbon nanotubes and the carrier gas descend to the bottom of the furnace, and the aggregates of the carbon nanotubes are wound up inside (see A in FIG. 1) or outside the furnace (see B in FIG. 1) to obtain a fiber. This method is advantageous over other methods in that carbon nanotube fibers can be mass produced at a spinning speed of a maximum of 20 to 30 m/min. However, the short retention time of the catalyst in the furnace makes it difficult to produce long, stable carbon nanotube fibers.